The water in a boiler may be 0.85. That happens at around 250 psig ( 17.2 Bar)
The column in the displacer chamber is sometimes a little cooler, but it IS warmed by continuous condensation of saturated steam in the top of the chamber. So it is not MUCH cooler, and the chamber is probably insulated.
The Masoneilan level transmitter works by displacement. The displacer weighs more when the chamber is empty than when the displacer is fully submerged.
SO if you calibrate it cold, the zero will be off lower than the level that corresponds to a zero signal in service. Then for every inch of travel of the liquid, you'll only get 85% of the buoyancy in service that cold water will give. So the unit's zero AND span will shift if you calibrate it cold and use it in a high-pressure (reduced SG) service. 11.9 inches of cold water corresponds to 14" of boiler water.
I have some experience with the Masoneilan units. The electronic unit has a SG scale in the instrument head used for calibration.
The pneumatic controllers have a scale inside the mechanism to allow for you to correct for SG changes.
Perhaps your intrument techs got confused and applied the correction backwards when they calibrated it?
In response to your second question: You will see the same effect with a DP instrument, and if one of the sensing legs sometimes gets cooler than the other you will have an intermittent calibration shift that will be hard to explain.
